Earthballs, Scleroderma sp.
Earthballs are a toxic fungi that are spherical in shape, with no defined stem. There are three types of Earthball found in the UK: the Common Earthball (Scleroderma citrinum), The Leopard Earthball (S. areolatum) and The Scaly Earthball (S. verrucosum). The one you're most likely to encounter is the Common Earthball, which occurs mostly in woodland, heathland and in short grassland habitats from July to November. All Earthballs have a thick, leathery skin, and insides that range from pink/purple, to black, depending on their maturity.
Possible Confusion & How to Stay Safe
At first glance, Earthballs look an awful lot like the edible puffballs, sharing the same shape and outside appearance. It's the inside however, that is very different. Puffballs are distinguished by being pure white on the inside, like a ball of fresh mozzerella cheese, whereas earthballs are always much darker in colour, often almost black. It's really important to always open-up a puffball to inspect the insides. If it's any other colour than pure white, then it's certainly not for eating.